1. Field of the Invention
The present invention generally relates to a vehicle-use map data processing device and method in which map data is externally acquired via a communication means, and in particular, to a device and method which can clear redundant map data at an appropriate timing.
2. Description of the Related Art
The use of on-vehicle map data processing devices is spreading, with the most common of these devices being a navigation device. A conventional navigation device comprises a memory medium, such as CD-ROM, for storing map data, and utilizes originally installed map data for navigation.
A new technology has been proposed and developed, in which a vehicle is connected to an information center via a communication means, and receives various useful information from the center via data communication. With this development as a background, offering map data to a vehicle via data communication is also proposed.
As shown in FIG. 1, an information center holds detailed map data for a wide area divided into a number of sectional maps. A vehicle acquires map data needed at that time from the information center. For example, a vehicle can acquire the map data of a sectional map covering the area around the present point, around the destination, or for any point designated by a user. Acquired map data (i.e., map data of maps A, B, C in FIG. 1) is held in the memory device (medium) on a vehicle side for use in navigation.
The advantage of this system lies in the fact that a vehicle need not hold large quantities of map data, and can nevertheless have access to a detailed map of a desired area. As an example, a vehicle need not store map data which the vehicle does not normally use, such as for maps around a sightseeing attraction because the vehicle can acquire map data concerning the attraction from an information center when it is actually needed.
Such a device is disclosed in, for example, Japanese Patent Laid-open No. Hei 7-262493. According to this publication, map data which a vehicle always carries is limited to that for basic maps showing main roads, and the like. The vehicle externally acquires map data of other detailed maps via communication with an external source.
Acquired map data via communication is stored in a memory device of a map data processing device. In other words, the memory device is accumulatively loaded with acquired map data. However, as the memory capacity of the memory device is limited, the quantity of map data which can be stored therein is naturally limited. Therefore, if map data kept in the memory device is not sometimes cleared to enlarge the vacancy, the stored map data overflows the memory capacity.
The easiest way of enlarging vacancy in the memory device is to delete some of the stored map data. However, deleted map data might be required later, and, in such a case, it might be necessary to externally acquire the same data again. Even more problematic, needed map data is not available for immediate use when needed because data transmission and subsequent processing must first be carried out. Moreover, extra communication costs may incur for reacquisition of map data. In light of this, reacquisition of map data is preferably avoided. However, no available conventional technology can meet this demand.
Also, with the device disclosed by the JP Laid-open No. Hei 7-262493, a user is required to designate map data to be deleted (delete object) and to then execute a clearance operation. In order to do this, the user must detect by himself which map data is stored in a memory device and which one of them is unnecessary. Such judgement may be hardly made by a user. Even if it is made, map data clearance is troublesome for a user to conduct. Also, unlike a PC user, a vehicle operator can not be expected to be familiar with file clearance operations, and it may be quite troublesome for such a user to perform a clearance operation by using a device such as is provided in a vehicle with a limited inside space, which is generally small. In this view, there is great desire for a device that can automatically and appropriately perform a data clearance operation on behalf of a user.